Methods and apparatus for improved reference signal correlation characteristics

ABSTRACT

The auto-correlation properties of a reference signal or pilot pattern, such as a position reference signal (PRS) in a Long Term Evolution communication system, is improved by modifying the currently specified PRS patterns, and/or by PRS pattern shaping. Pattern shaping can result in creation of virtual PRS patterns, for example, by controlling the PRS transmitted or received power used by the correlator. PRS power shaping can be implemented differently according to the location where the PRS power is calculated, e.g., in a network node or in a user equipment.

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 61/295,846 that was filed on Jan. 18,2010, and that is incorporated in this application by reference.

TECHNICAL FIELD

This invention relates to wireless communications networks and moreparticularly to transmitted signal pattern design and wireless networkarchitectures that utilize signal measurements from multiple cells forpositioning, location and location-based services.

BACKGROUND

According to the Third Generation Partnership Project (3GPP)specifications for wireless communication systems (Release 8 and laterReleases), a Long Term Evolution (LTE) communication system usesorthogonal frequency division multiplex (OFDM) as a multiple accesstechnique (called OFDMA) in the downlink (DL) from system nodes to userequipments (UEs). High-Speed Packet Access (HSPA) and early versions ofLTE are sometimes called “third generation” (3G) communication systems.LTE-Advanced (Release 10 and later) has been ratified as a “fourthgeneration” (4G) communication system. The LTE specifications can beseen as an evolution of current wideband code division multiple access(WCDMA) specifications. The 3GPP promulgates specifications for LTE,HSPA, WCDMA, and other communication systems.

LTE communication channels are described in 3GPP Technical Specification(TS) 36.211 V9.1.0, Physical Channels and Modulation (Release 9)(December 2009) and other specifications. For example, controlinformation exchanged by evolved NodeBs (eNodeBs) and user equipments(UEs) is conveyed by physical uplink control channels (PUCCHs) and byphysical downlink control channel (PDCCHs). In an OFDMA communicationsystem, a data stream to be transmitted is portioned among a number ofnarrowband subcarriers that are transmitted in parallel. In general, aphysical resource block is a particular number of particular subcarriersused for a particular period of time. Different groups of subcarrierscan be used at different times for different purposes and differentusers. OFDMA communication systems are described in the literature, forexample, U.S. Patent Application Publication No. US 2008/0031368 A1 byB. Lindoff et al.

The possibility of identifying user geographical location, or position,in a system has enabled a large variety of commercial and non-commercialservices, e.g., navigation assistance, social networking, location-awareadvertising, emergency calls, etc. Different services can have differentpositioning accuracy requirements imposed by the application. Inaddition, some regulatory requirements on the positioning accuracy forbasic emergency services exist in some countries, e.g., FCC E911 in theU.S., which puts an extra burden on the desired quality of thepositioning service.

FIG. 1A illustrates a user plane of an exemplary positioningarchitecture in an LTE cellular communication system 100 that includesUEs 110, 120, a radio access network (RAN) that includes a plurality ofeNodeBs 130-1, 130-2, . . . , 130-N, and a core network that includes aserving gateway (SGW) node 140 and a packet data network 150. The system100 also includes a positioning node 160, which in the user plane iscalled a Secure user-plane Location (SUPL) Platform (SLP). In the userplane of a positioning architecture, the UEs 110, 120 are more preciselycalled SUPL enabled terminals (SETs).

Each eNodeB 130-1, 130-2, . . . , 130-N serves a respective geographicalarea that is divided into one or more cells. An eNodeB can use one ormore antennas at one or more sites to transmit information into itscell(s), and different antennas can transmit respective, different pilotand other signals. Neighboring eNodeBs are coupled to each other by anX2-protocol interface that supports active-mode mobility of the UEs. AneNodeB controls various radio network functions, including for examplesingle-cell radio resource management (RRM), such as radio access bearersetup, handover, UE uplink/downlink scheduling, etc. Multi-cell RRMfunctions can also use the X2-protocol interfaces. Each eNodeB alsocarries out the Layer-1 functions of coding, decoding, modulating,demodulating, interleaving, de-interleaving, etc., and the Layer-2retransmission mechanisms, such as hybrid automatic repeat request(HARQ). The eNodeBs 130-1, 130-2, . . . , 130-N are coupled to one ormore SGWs 140 (only one of which is shown in FIG. 1A).

FIG. 1B illustrates a control plane of the exemplary positioningarchitecture in the LTE communication system 100. In the control planeas shown, an LTE-Uu protocol interface couples the UE 110 to the eNodeB130, and an S1-MME protocol interface couples the eNodeB 130 to aMobility Management Entity (MME) 140, which is a name for the SGW in thecontrol plane. The positioning node 160 is called an evolved ServingMobile Location Center (E-SMLC) in the control plane, and is coupled tothe MME 140 by a signaling link selection (SLs) protocol interface. Itwill be understood that there can be a communication interface betweenthe SLP and E-SMLC for interworking in the positioning node 160. 3GPPhas standardized two protocols specifically to support positioning inLTE: an LTE Positioning Protocol (LPP) and an LTE Positioning ProtocolAnnex (LPPa). Messaging according to those protocols is also depicted inFIG. 1B.

The LPP is a point-to-point protocol between a location services (LCS)server, such as the E-SMLC 160, and a LCS target device, such as the UE110, that is used to position the target device. Transmitted LPPmessages are transparent to an MME 140, and use radio resource control(RRC) protocol messages for transport over an LTE-Uu interface betweenthe UE 110 and the eNodeB 130, and then S1 application protocol (S1AP)messages over the S1-MME interface between the eNodeB 130 and the MME140, and then LCS-AP messages over the SLs interface between the MME 140and the E-SMLC 160. LPP is defined in 3GPP TS 36.355 V9.2.1, LTEPositioning Protocol (LPP) (Release 9) (June 2010), for example.

LPPa is a protocol for an interface between an eNodeB and a positioningserver, such as the E-SMLC 160. LPPa messages are also transparent tothe MME 140, which routes LPPa message packets over the S1-MME and SLsinterfaces without knowledge of the involved LPPa transactions. LPPa isspecified only for control-plane positioning procedures, but with userplane/control plane interworking, LPPa can also assist the user plane byquerying eNodeBs for information and eNodeB measurements not related toa UE connection. LPPa is defined in 3GPP TS 36.455 V9.2.0, LTEPositioning Protocol A (LPPa) (Release 9) (June 2010), for example.

In the user-plane positioning architecture, the SUPL service usesestablished data-bearing channels (i.e., the LTE user plane) andpositioning protocols (i.e., LPP) for exchanging the positioning-relateddata between a LCS target (e.g., a SET 110, 120) and a LCS server (e.g.,a SLP 160).

UEs 110, 120 are generally wireless communication devices that can becellular radiotelephones, personal digital assistants (PDAs), PersonalCommunications System (PCS) terminals, laptop computers, palmtopcomputers, or any other type of device or appliance that includes acommunication transceiver that permits the device to communicate withother devices via a wireless link. A PCS terminal can combine a cellularradiotelephone with data processing, and facsimile and datacommunication capabilities. A PDA can include a radiotelephone, a pager,an Internet/intranet access device, a web browser, an organizer,calendars, and/or a global positioning system (GPS) receiver. One ormore of UEs 110, 120 can be referred to as a “pervasive computing”device. In some implementations, the UEs 110, 120 can include wirelinetelephones (e.g., Plain Old Telephone system (POTs) telephones) that areconnected to a Public Switched Telephone Network (PSTN). In apositioning architecture like that depicted in FIG. 1A, a UE can also bea base station, signal relay, radio repeater, sensor, etc.

As described in 3GPP TS 36.305 V9.3.0, Stage 2 Functional Specificationof User Equipment (UE) Positioning in E-UTRAN (Release 9) (June 2010),for example, the positioning node 160 can determine the geographicpositions of UEs in the system 100 in a wide variety of ways, e.g.,Global Navigation Satellite System (GNSS), Observed Time Difference OfArrival (OTDOA), Uplink Time Difference Of Arrival (UTDOA), EnhancedCell ID (E-CID), radio fingerprinting, etc. GNSS is a generic name forsatellite-based positioning systems with global coverage. Examples ofGNSS systems include the U.S. GPS, the European Galileo, the RussianGlonass, and the Chinese Compass. With GNSS, a position is typicallyobtained by triangulation based on measurements of times of arrival ofreference signals. OTDOA uses timing measurements conducted on downlink(DL) reference signals received from multiple locations, and UTDOA usestiming measurements performed on UL reference signals received atmultiple locations.

In OTDOA and UTDOA, the position is obtained by multi-lateration ortriangulation based on intersections of hyperbolas or circles. Methodsbased on multi-lateration, which is a way to determine a geometricalposition from intersection of multiple surfaces, e.g., spheres orhyperboloids, require measurements from multiple sites, such as eNodeBantennas, with a good geometry; ideally at least three such sites arenecessary for a two-dimensional (2D) position and four sites for a threedimensional (3D) position, which in practice means that a UE needs tomeasure significantly more cells, also because some of them areco-located.

In radio fingerprinting positioning, the positioning node 160 usesinformation in a radio fingerprint database that stores radiofingerprints derived from Evolved Universal Terrestrial Radio AccessNetwork (E-UTRAN) and/or Inter-Radio Access Technology (IRAT)measurement data. The E-UTRAN and/or IRAT measurement data can beprovided to the positioning node 160 in conjunction with precisegeographic position data obtained at the same geographic location atwhich the E-UTRAN and/or IRAT measurements were performed (e.g., GPSposition data). The positioning node 160 can subsequently receiveE-UTRAN and/or IRAT radio fingerprint measurement data from UEs 110, 120and perform a lookup in the radio fingerprint database to identify astored radio fingerprint that matches the received E-UTRAN and/or IRATradio fingerprint measurement data, and to retrieve a stored geographicposition that corresponds to the matching radio fingerprint. Thepositioning node 160 can provide this geographic position to the UE thatsent the radio fingerprint measurement data, or to other destinations,such as, for example, an emergency or police call center.

The network 100 can exchange information with one or more other networksof any type, including a local area network (LAN); a wide area network(WAN); a metropolitan area network; a telephone network, such as apublic switched terminal network or a public land mobile network; asatellite network; an intranet; the Internet; or a combination ofnetworks. It will be appreciated that the number of nodes illustrated inFIG. 1 is purely exemplary. Other configurations with more, fewer, or adifferent arrangement of nodes can be implemented. Moreover, one or morenodes in FIG. 1 can perform one or more of the tasks described as beingperformed by one or more other nodes in FIG. 1. For example, parts ofthe functionality of the eNodeBs can be divided among one or more basestations and one or more radio network controllers, and otherfunctionalities can be moved to other nodes in the network.

FIG. 2 is a frequency-vs.-time plot showing an arrangement of downlink(DL) subcarriers, or tones, in an LTE system. In general as specified in3GPP 36.211, DL signals in the frequency division duplex (FDD) mode ofLTE are organized into successive frames of 10 milliseconds (ms)duration. Each frame is divided into ten successive subframes, and eachsubframe is divided into two successive time slots of 0.5 ms. Each slotincludes either three, six or seven OFDM symbols, depending on whetherthe symbols include long (extended) or short (normal) cyclic prefixes.An LTE physical resource block (RB) comprises a group of resourceelements (REs) spanning twelve consecutive subcarriers in the frequencydomain and one time slot in the time domain. A physical RB isillustrated by the shaded area in FIG. 2 for symbols having a normalcyclic prefix. The subcarriers are spaced apart by fifteen kilohertz(kHz) and together occupy approximately 180 kHz in frequency. In anEvolved Multicast Broadcast Multimedia Services (MBMS) Single FrequencyNetwork (MBSFN), the subcarriers are spaced apart by either 15 kHz or7.5 kHz. A RE spans one subcarrier (frequency domain) and one symbol(time domain). It will be understood that RBs could include othernumbers of subcarriers for other periods of time in other communicationsystems.

In the case of OFDM transmission, an eNodeB transmits reference signalscomprising known reference symbols on known subcarriers in the OFDMfrequency-vs.-time grid. For example, cell-specific reference signals(CRS) are described in Clauses 6.10 and 6.11 of 3GPP TS 36.211 V9.0.0,Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channelsand Modulation (Release 9) (December 2009). A UE uses its receivedversions of the known reference signals to estimate characteristics,such as the impulse response, of its DL channel. The UE can then use theestimated channel matrix for coherent demodulation of the received DLsignal, and obtain the potential beam-forming gain, spatial diversitygain, and spatial multiplexing gain available with multiple antennas. Inaddition, the reference signals can be used to do channel qualitymeasurement to support link adaptation.

Up to four CRS corresponding to up to four transmit antennas of aneNodeB are currently specified, and FIG. 3A shows the arrangement ofreference symbols in a subframe for one antenna, FIG. 3B shows thearrangement of reference symbols in a subframe for two antennas, andFIGS. 3C, 3D depict the arrangement of reference symbols in a subframefor four antennas.

FIG. 3A shows a frequency-vs.-time grid that includes reference symbolsR₀ that are transmitted at known subcarrier and time symbols in asubframe from an eNodeB having one antenna port 0. In FIG. 3A, thereference symbol R₀ is depicted as transmitted on every sixth subcarrierin OFDM symbol 0 and OFDM symbol 4 in every seven-symbol (normal cyclicprefix) time slot. Also, the reference symbols R₀ in OFDM symbol 4 areoffset by three subcarriers relative to the reference symbols in OFDMsymbol 0, the first OFDM symbol in a slot. It should be understood thatthe reference symbols R₀ can be transmitted in other OFDM symbolsdepending on whether the symbols have long or short cyclic prefixes. Forexample, the reference symbols R₀ can be transmitted in OFDM symbol 3when the OFDM symbols have long cyclic prefixes.

FIG. 3B shows frequency-vs.-time grids that include reference symbols R₀that are transmitted at known frequencies and time instants in asubframe from an antenna port 0 (which is the same as FIG. 3A) andreference symbols R₁ that are transmitted at known frequencies and timeinstants in a subframe from an antenna port 1. Cross-hatched REsindicate reference symbols that are not transmitted by a particularantenna port.

FIGS. 3C and 3D show frequency-vs.-time grids that include referencesymbols R₀ from an antenna port 0 (which is the same as FIG. 3A),reference symbols R₁ that are transmitted from an antenna port 1 (whichis the same as FIG. 3B), reference symbols R₂ that are transmitted froman antenna port 2, and reference symbols R₃ that are transmitted from anantenna port 3. As in FIG. 3B, cross-hatched REs in FIGS. 3C, 3Dindicate reference symbols that are not transmitted by a particularantenna port. It will be noted in FIG. 3D that the reference symbols R₂,R₃ are depicted as transmitted in OFDM symbols 1, 5, respectively, inevery seven-symbol time slot.

Some communication systems, such as LTE-Advanced, can employ more thanfour transmit antennas in order to achieve better performance. Forexample, a system having eNodeBs with eight transmit antennas will needextensions of the LTE CRS signals described above.

To enable positioning in LTE and facilitate positioning measurements ofa proper quality and for a sufficient number of distinct locations, newphysical signals dedicated for positioning, called positioning referencesignals (PRS), have been introduced and specific positioning subframeshave been agreed in 3GPP, although the existing CRS described above canin principle also be used for positioning.

PRS and Positioning Subframes in LTE

PRS are transmitted from one antenna port (R6) according to apre-defined pattern, as described for example in Clause 6.10.4 of 3GPPTS 36.211 V9.0.0, Evolved Universal Terrestrial Radio Access (E-UTRA),Physical Channels and Modulation (Release 9) (December 2009). One of thecurrently agreed PRS patterns is shown in FIG. 4, which corresponds tothe left-hand side of FIG. 6.10.4.2-1 of 3GPP TS 36.211, where the greysquares indicate PRS resource elements that include reference symbols R₆from an antenna port 6 within an RB in frequency and one subframe intime with the normal cyclic prefix. A physical broadcast channel (PBCH)can be transmitted from one or two antenna ports in the RBs.

A set of frequency shifts can be applied to the pre-defined PRS patternsto obtain a set of orthogonal patterns which can be used in neighborcells to reduce interference on the PRS and thus improve positioningmeasurements. The effective frequency reuse of six can be modeled inthis way. The frequency shift is defined as a function of Physical CellID (PCI) as follows:v _(shift)=mod(PCI,6)in which v_(shift) is the frequency shift, mod( ) is the modulofunction, and PCI is the Physical Cell ID. The PRS can also betransmitted with zero power, or muted.

To improve hearability of the PRS, i.e., to enable a UE to detect thePRS from multiple sites and with a reasonable quality, positioningsubframes have been designed as low-interference subframes, i.e., it hasalso been agreed that no data transmissions are allowed in general inpositioning subframes, although a network can at its own risk stillallow some DL transmission in positioning subframes. As a result,synchronous networks' PRS are ideally interfered with only by PRS fromother cells having the same PRS pattern index, i.e., the same frequency(vertical) shift (v_(shift)), and not by data transmissions.

In partially aligned asynchronous networks, PRS can still be interferedwith by transmissions over data channels, control channels, and anyphysical signals when positioning subframes collide with normalsubframes, although the interference is reduced by the partialalignment, i.e., by aligning the beginnings of positioning subframes inmultiple cells within one-half of a subframe with respect to some timebase. PRS are transmitted in pre-defined positioning subframes groupedby several consecutive subframes (N_(PRS)), i.e., one positioningoccasion, which occur periodically with a certain periodicity of Nsubframes, i.e., the time interval between two positioning occasions.The currently agreed periods N are 160, 320, 640, and 1280 ms, and thenumber of consecutive subframes N_(PRS) can be 1, 2, 4, or 6, asdescribed in 3GPP TS 36.211 cited above.

With the cell PCI, the PRS configuration (comprising the offset fromsystem frame number (SFN) 0, periodicity, and the number of positioningsubframes) is signaled to the UE as a part of the OTDOA assistance datafrom the positioning server (e.g., an E-SMLC) to a positioning target(e.g., a UE) using the LPP protocol. The PRS pattern for PRS resourceelements in the time-frequency domain, as described above, can be foundout by the UE from the cell PCI.

Correlation Properties of Reference Signal Patterns

Signals, including reference signals used for positioning (PRS or CRS inLTE), typically do not have ideal correlation (auto- andcross-correlation) properties. Better auto-correlation propertiesenhance the resolvability of multipath, which is very important, forexample, in urban environments where OTDOA is expected to complementAssisted Global Positioning System (A-GPS). Poor auto-correlationproperties may also affect the search window size, which is used toidentify the PRS pattern by detecting the correlation peak. Forinstance, poor auto-correlation properties of the PRS pattern constrainthe UE to use a more precise search window in order to avoid searchingfor the unnecessary correlation peaks (i.e., side lobes). On the otherhand, the search window depends on the UE location uncertainty. Ideally,the maximum search window is defined by a range [−r, r], where r is themaximum cell range.

A general observation known in the art is that the best auto-correlationproperties are achieved when the signal is transmitted over allsubcarriers (with uniform sum energy density over the subcarriers)during a coherent time interval, although not necessarily on allsubcarriers in each OFDM symbol. This is because the auto-correlation ofa periodic function is, itself, periodic with the same period, whichmeans that the presence of a periodic component in the pattern maygenerate side-lobe auto-correlation peaks.

Transmitting the signal over the entire bandwidth in a symbol is not agood approach in a synchronous network from an interference-managementpoint of view, unless predefined time-offsets are applied in differentcells to mimic frequency reuse. This means that it is preferable totransmit reference signals according to a predefined pattern.Furthermore, a higher frequency reuse is desirable in networks where theinterference is crucial (e.g., with high load, short inter-sitedistance, etc.). Such a sparseness property is enjoyed, for example, bypatterns designed based on Costas arrays, which are traditionally usedin sonar and radar communication. A Costas array is a geometrical set ofn points lying on the squares of a n×n checkerboard, such that each rowor column contains only one point, and that all of the n(n−1)/2displacement vectors between each pair of dots are distinct. Inpractice, however, it is not always possible to achieve patterns withoptimal correlation properties.

The frequency-time transmission patterns for PRS currently defined by3GPP TS 36.211 have correlation properties that can be insufficientlygood for positioning in a “rich” multipath environment, i.e., anenvironment with a large number of multipath signals. As an illustrativeexample, FIGS. 5A, 5B show the correlator outputs for twenty-five PRSRBs with normal and extended CP, respectively, versus time shown inmeters (converted by multiplying time by the speed of light in metersper second). Periodic strong side lobes can be seen, for example, atabout 3 km, 6 km, and so on.

Furthermore, there is no possibility to control PRS power within asubframe, because, as stated in Clause 5.2 of 3GPP TS 36.213 V9.3.0,Evolved Universal Terrestrial Radio Access (E-UTRA), Physical LayerProcedures (Release 9) (September 2010): “A UE may assume that downlinkpositioning reference signal EPRE [energy per resource element] isconstant across the positioning reference signal bandwidth and acrossall OFDM symbols that contain positioning reference signals in a givenpositioning reference signal occasion.”

SUMMARY

The auto-correlation properties of a reference signal or pilot pattern,such as a PRS, can be improved in hard and/or soft ways, which is tosay, respectively, by modifying the currently specified PRS patterns,and/or by PRS pattern shaping (i.e., creating virtual PRS patterns),e.g., by controlling the PRS transmitted power or the received powerused as input to the correlator. PRS power shaping can be implementeddifferently according to the location where the PRS power is calculated,e.g., in a network node or in a UE.

According to an aspect of this invention, there is provided a method ofusing reference signals (RS) in an orthogonal frequency divisionmultiplex communication system in which the RS are organized in a RSpattern of resource elements (REs) that includes a first plurality ofcolumns corresponding to symbols and a second plurality of rowscorresponding to subcarriers. The method includes forming a modified RSpattern based on a predetermined RS pattern by at least one of:cyclically shifting REs in at least one column of the predetermined RSpattern; assigning respective different transmission power levels toselected REs of the predetermined RS pattern; and adjusting a receivedsignal power of selected REs of the predetermined RS pattern.

According to another aspect of this invention, there is provided areference signal generator in an orthogonal frequency division multiplexcommunication system in which reference signals (RS) are organized in aRS pattern of resource elements (REs) that includes a first plurality ofcolumns corresponding to symbols and a second plurality of rowscorresponding to subcarriers and/or that includes a set of power levelsof RS REs. The generator includes an electronic processor configured toform a modified RS pattern based on a predetermined RS pattern by atleast one of: cyclically shifting REs in at least one column of thepredetermined RS pattern; assigning respective different transmissionpower levels to selected resource elements of the predetermined RSpattern; and adjusting a received signal power of selected REs of thepredetermined RS pattern.

According to another aspect of this invention, there is provided anapparatus for a user equipment in an orthogonal frequency divisionmultiplex communication system for using reference signals (RS)organized in a RS pattern of resource elements (REs). The apparatusincludes a correlator; a modified RS pattern generator configured togenerate a modified RS pattern based on a predetermined RS pattern thatis included in a received signal, where the modified RS pattern includesselected REs of the predetermined RS pattern that have adjusted signalpower levels; and an electronic processor configured to modify thereceived signal according to the modified RS pattern and form a modifiedreceived signal. The correlator is configured to correlate the modifiedreceived signal and the modified RS pattern and form a correlationresult.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects, features, and advantages of this invention will beunderstood by reading this description in conjunction with the drawings,in which:

FIG. 1A illustrates a user plane of a positioning architecture in a LongTerm Evolution communication system;

FIG. 1B illustrates a control plane of a positioning architecture in aLong Term Evolution communication system;

FIG. 2 is a frequency-vs.-time plot showing an arrangement of downlinksubcarriers in a Long Term Evolution communication system;

FIGS. 3A, 3B, 3C, 3D show frequency-vs.-time grids that includereference symbols transmitted from different antenna ports;

FIG. 4 shows a conventional positioning reference signal pattern;

FIGS. 5A, 5B depict auto-correlation properties of conventionalpositioning reference signal patterns, including the pattern shown inFIG. 4;

FIGS. 6A, 6B, 6C show examples of modified positioning reference signalpatterns;

FIGS. 7A, 7B depict auto-correlation properties of the reference signalpatterns shown in FIGS. 6A, 6B, 6C;

FIG. 8 shows a positioning reference signal pattern;

FIGS. 9A, 9B, 9C are flow charts of methods of generating referencesignals;

FIG. 10 is a block diagram of an example of a portion of transmitter fora communication system; and

FIG. 11 is a block diagram of an arrangement in a user equipment.

DETAILED DESCRIPTION

This description is written in terms of an LTE communication system butthe artisan will understand that this invention can be embodied in otherkinds of communication system. It will also be understood that thisinvention is not limited to improvement of only positioning referencesignals but includes any type of reference signal or pilot pattern.

Although it is not always possible to achieve signal patterns withoptimal correlation properties, this invention enables methods andapparatus to optimize a pattern. The auto-correlation properties of PRScan be improved in hard and/or soft ways, which is to say, respectively,by modifying the currently specified PRS patterns, and/or by PRS patternshaping (i.e., creating virtual PRS patterns), e.g., by controlling thePRS transmitted or received power. Optimization can be done with respectto different objectives. In timing-measurement-based positioning, it isimportant to avoid secondary correlation peaks, especially in theproximity of the main peak because otherwise a correlation side lobe canbe mistakenly interpreted as the main peak, which results in a falsealarm.

In accordance with this invention, pattern optimization can beimplemented by modifying a currently specified PRS pattern and/or byshaping a currently specified PRS pattern. Pattern shaping can beaccomplished by controlling the PRS transmitted power level and/or thePRS received power level, thereby creating in effect one or more virtualPRS patterns. It will be understood, of course, that pattern shaping isalso a type of pattern modification. To support PRS pattern shaping byUEs, an eNodeB can signal its PRS transmit power level to the UE overthe radio interface according to either the LTE positioning protocol(LPP) or the radio resource control (RRC) protocol, and the neededinformation can be included in the OTDOA assistance data. In the case ofthe same PRS power per RE or per subcarrier (since the UE knows the PRSpattern), this PRS transmit power level information is not needed,unless muting is applied.

As described in more detail below, PRS power shaping can be implementeddifferently according to the location where the PRS power is calculated.In one embodiment, PRS pattern shaping is performed in the network,i.e., the PRS transmit power is set by a network node, such as an eNodeBor a positioning node (e.g., an eSMLC in the control plane in LTE,and/or another entity located inside or outside an eSMLC but with aproprietary interface, which may be a part of a user-plane positioningsolution). In another embodiment, the UE reshapes its received signalsbefore the correlator in its receiver.

Currently Specified PRS Pattern Modification

Currently specified PRS patterns can be improved by applying a strategythat includes one or more of the following rules: fill up emptysubcarriers with at least one PRS RE; slightly rearrange PRS REs suchthat the PRS RE density over coherently accumulated segments (e.g., allsymbols within a RB) is more uniform and preferably as uniform aspossible; and maintain the same frequency reuse as in the standardizedpatterns over all symbols where PRS is transmitted. The last of therules is currently believed to be important because, among other things,it minimizes changes to frequency allocation in a network and sofacilitates implementation of this invention in existing and futurenetworks. Moreover, modification of the currently specified signalpatterns according to the rules can be realized by shifting REs in someof the columns in the PRS patterns in a cyclic way. For a cyclic shift,in other words, the same number of REs per column and the space betweenthem (in a cyclic sense) in a currently standardized pattern arepreserved in a modified pattern.

FIGS. 6A, 6B, and 6C show examples of modified patterns of referencesymbols R₆ from antenna port 6 for normal and extended CP, respectively.Each of FIGS. 6A, 6B corresponds to the left-hand and right-hand sidesof FIG. 6.10.4.2-1 of 3GPP TS 36.211. FIG. 6C corresponds to theleft-hand and right-hand sides of FIG. 6.10.4.2-2 of 3GPP TS 36.211. PRSpatterns following the above strategy are modified versions of thepatterns specified by 3GPP TS 36.211, but they have the advantage ofmuch better correlation properties, as illustrated by FIGS. 7A, 7B.

In FIGS. 6A, 6B, the modifications are obtained by shifting REs in thetwo last columns in opposite frequency directions by different numbersof subcarriers. The crossed squares are the PRS REs in the modifiedcolumns as they are in the current pattern, and the dots indicate thenew PRS REs obtained by cyclic shifts in those columns. The dots in FIG.6A are the shifted PRS REs showing a currently preferred solution inwhich OFDM symbol 5 in odd-numbered slots is shifted down onesub-carrier and OFDM symbol 6 in the same slots is shifted up twosubcarriers. The dots in FIG. 6B are the shifted PRS REs showing acurrently less preferred solution, in which OFDM symbol 5 inodd-numbered slots is shifted up one subcarrier, although it has thesame correlation properties as the solution with dots depicted in FIG.6A. A difference between the two solutions in FIGS. 6A, 6B is that thediagonal structure of the patterns is preserved with the latter, whilethe former is better protected against possible inter-symbolinterference with CRS REs transmitted in the symbol to the left (CRS REsoccur in crosses of empty rows and columns, except the first threecolumns (OFDM symbols) in even-numbered slots that are reserved for thecontrol region).

As an example of a “cyclic” shift, the distance between the lower dotand the upper dot in the last column is six subcarriers (countingupward), but the distance can also be found for those two dots bycounting upward from the upper dot to the top of the subframe (i.e., +3)and then from the bottom of the subframe upward to the lower dot (i.e.,+3), which also totals to six. Conveniently, the total number ofsubcarriers is twelve, and so both counting methods yield the sametotal: six, but even for spacings of 5 and 7, with any cyclic shift thetwo distances between the REs are preserved (in a cyclic sense) sincethe relative positions of REs within a column are unchanged after acyclic shift.

FIGS. 7A, 7B depict the improved auto-correlation properties of themodified patterns shown in FIGS. 6A, 6B, 6C. FIGS. 7A, 7B show thecorrelator outputs for twenty-five PRS RBs with normal and extended CP,respectively, versus time shown in meters (converted by multiplying timeby the speed of light in meters per second). Side lobes at about 3 kmare absent, although side lobes at about 6 km are seen. Asaforementioned, the PRS patterns in FIGS. 6A, 6B have similarauto-correlation properties, which are represented in FIG. 7A. Thepresented results in FIGS. 7A, 7B are for only one and two PBCH antennaports. The differences between FIGS. 7A, 7B and FIGS. 5A, 5B aresignificant.

PRS Pattern Shaping

Consider a receiver that includes a correlator, such as a matchedfilter, a shift register, etc., that generates the auto-correlation of areceived predetermined signal, assuming an ideal noise-free channel.Given a correlator output R in the frequency domain, the equivalentoutput r in the time domain, which can be produced with an inversediscrete Fourier transform (Inverse DFT or IDFT), is given by thefollowing expression:

$\begin{matrix}{r_{n} = {\frac{1}{\sqrt{N}}{\sum\limits_{k = 0}^{N - 1}{R_{k}{{\mathbb{e}}^{{j2\pi}\;{{nk}/N}}.}}}}} & (1)\end{matrix}$in which n is a time-sample index for time-domain correlation coefficentr_(n), k is a frequency-sample index for frequency-domain correlationcoefficient R_(k), and N is the total number of IDFT samples. Note thatthe definition in Eq. 1 has a periodicity of N, which is to say:X _(k+N) =X _(k),X _(n+N) =X _(n).  (2)

By Parseval's theorem, the sum (or integral) of the square of a functionis equal to the sum (or integral) of the square of its Fouriertransform, and so the DFT relates the time- and frequency-domain energyas follows:

$\begin{matrix}{{{\sum\limits_{n = 0}^{N - 1}{x_{n}}^{2}} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{X_{k}}^{2}}}},} & (3)\end{matrix}$where x_(n) is the n-th element of the (time domain) sequence x=(x₀, . .. , x_(n), . . . , x_(N−1)) of N samples, X_(k) is the k-th element ofthe (frequency domain) sequence X=(X₀, . . . , X_(k), . . . , X_(N−1))of N samples, which is the DFT of x. Applying the theorem to thecorrelator output, we get the following expression:

$\begin{matrix}{{\sum\limits_{n = 0}^{N - 1}\left( r_{n} \right)^{2}} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{\left( R_{k} \right)^{2}.}}}} & (4)\end{matrix}$in which the parameters and variables are as described above.

Furthermore, from the Cauchy-Schwarz inequality, the continuousautocorrelation function reaches its peak at the origin, where it takesa real value. The same result holds in the discrete case, e.g.,|R_(k)|≦R₀,k=0, . . . , K.

From the positioning point of view, we are interested in getting themain peak as high as possible compared to other peaks in the correlatoroutput (ideally, there is only one peak). So, one can formulate anoptimization problem to maximize the squared power of the main peak overthe total energy, i.e.:

$\begin{matrix}\left. \frac{\left( {\max\limits_{n}r_{n}} \right)^{2}}{\sum\limits_{n = 0}^{N - 1}\left( r_{n} \right)^{2}}\rightarrow{\max.} \right. & (5)\end{matrix}$in which the parameters and variables are as described above.

The maximum value the ratio in Eq. 5 can take is unity, otherwise it isin the open range (0,1]. Another alternative is to consider in thenominator of Eq. 5 not the power of the main peak but a metricreflecting accumulated energy in the correlation peak over a number ofrelevant samples, say, 2(n₀+1) samples with indexes in a range +/−n₀relative to the main peak (zero for the main peak), e.g.:

$\begin{matrix}\left. \frac{\sum\limits_{{- n_{0}} \leq n \leq n_{0}}r_{n}^{2}}{\sum\limits_{n = 0}^{N - 1}\left( r_{n} \right)^{2}}\rightarrow{\max.} \right. & (6)\end{matrix}$in which the parameters and variables are as described above. Thecorrelation coefficients r depend on transmit PRS power, PRS sequence,and the channel.

It is known that the spectrum of the autocorrelation of a signal isidentical to the power spectrum of the signal, normalized by theintegration period N, which is to say:

$\begin{matrix}{R_{k} = {\frac{{X_{k}}^{2}}{N}.}} & (7)\end{matrix}$Eq. 7, however, does not cover the case when the signal X_(k) istransmitted over several symbols. PRS signals typically span overseveral OFDM symbols, so for multiple coherently accumulated symbols,auto-correlation coefficients can be written as follows:

$\begin{matrix}{{R_{k} = {\frac{1}{N}{\sum\limits_{l = 0}^{L - 1}{X_{l,k}}^{2}}}},} & (8)\end{matrix}$where X_(l,k) is the received signal in symbol l and subcarrier k, and Lis the number of such symbols where PRS is transmitted (e.g., PRSpattern width measured in symbols). With ideal receivers, the receivedsignal is X_(l,k)=H_(l,k)S_(l,k), where S_(l,k) is the transmittedsequence, and H_(l,k) is the communication channel's impulse response.With the signal property |S_(k)|=1, the frequency-domainauto-correlation coefficients R_(k) given by Eq. 8 can be written as:

$\begin{matrix}{R_{k} = {\frac{1}{N}{\sum\limits_{l = 0}^{L - 1}{{H_{l,k}}^{2}.}}}} & (9)\end{matrix}$

Since we are interested in the PRS pattern properties withoutrestricting to any specific channel, we can assume a unit channeldecoupled from the transmit PRS power. With this assumption, Eq. 9reduces to a scaled sum of transmit PRS powers, which is given by:

$\begin{matrix}{R_{k} = {\frac{1}{N}{\sum\limits_{l = 0}^{L - 1}P_{l,k}}}} & (10)\end{matrix}$in which P_(l,k) is the transmit power of symbol I and subcarrier k.Those skilled in this art will understand that this description is notlimited to any specific algorithms for computing DFTs and IDFTs.Suitable algorithms can be based on the fast Fourier transform, a directtransform, or any other approach.PRS Power Optimization Problem

Now we can formulate a complete optimization problem, where theobjective is to find PRS power allocation (unknown variables of theoptimization model) by REs in the PRS REs. The optimization problemcomprises an optimization objective (e.g., formulated in Eqs. 5 or 6),the relation between R_(k) and r_(n) modeled by Eq. 1, and the equationfor computing the denominator in the objective function from Eq. 4. Notethat with the aforementioned relations, it is not even necessary toapply an inverse fast Fourier transform (IFFT), since we can utilize theknown properties to find the relation between the main peak (e.g., poweror energy) and the total energy. The problem is a non-linearoptimization problem. More constraints can be added to limit the set offeasible solutions. For example, if a PRS pattern is defined for a smallsubset of subcarriers and then repeated in the frequency domain in allphysical resource blocks (PRBs) within the PRS bandwidth, this needs tobe taken into account too by introducing a relevant set of constraints.

In one embodiment, to reduce the problem complexity and so the networkand UE implementation complexity, it is possible to assume, for example,some pre-defined power split between REs on the same subcarrier, i.e.,to define a unique mapping between P_(l,k) and R_(k). For example, iftwo PRS REs are allowed on subcarrier k in a PRS pattern, then the totalpower on the subcarrier over these RE defined by Eq. 10 can be equallysplit between the two PRS REs.

An application of such an approach is depicted in FIG. 8A, which showsan optimized PRS pattern on antenna port R₆. In some subcarriers(horizontal rows), there are two PRS REs (indicated by circling, such asthe third row from the top), and in some subcarriers there is only onePRS RE (such as the second and fourth rows from the top). With an equaltotal power allocation per subcarrier, we get good auto-correlationproperties, similar to what is shown in FIG. 7B (with a possibledifference in the absolute values), but this implies that the PRS powerper PRS RE need not be uniform over all PRS REs in the pattern, andthere are many designs possible to achieve a given power vectorcorresponding to a given vector R_(k). With an equal-split powerapproach, the two-PRS-RE subcarriers (i.e., the PRS REs on thesesubcarriers are gray squares marked R6 in FIG. 8A) get allocated power afactor of two lower compared to the one-PRS-RE subcarriers (i.e., thePRS REs on these subcarriers are white squares marked R6 in FIG. 8A).

In another embodiment of the invention, when there is at least one PRSallowed in each subcarrier, a virtual pattern based on a Costa array iscreated by assigning zero power in all PRS REs, except one, in eachsubcarrier such that only one non-zero-power PRS RE is present in eachsymbol in the PRS pattern. The virtual pattern has one non-zero-powerPRS RE per subcarrier. For example, in each subcarrier with gray squaresin the pattern depicted in FIG. 8A, one of the gray squares R6 is“transmitted” with zero power. Geometrically, a Costas array can beviewed as a set of n points lying on the squares of an n×n checkerboard,such that each row or column contains only one point. This property isachieved in the virtual pattern over the symbols where PRS can betransmitted. Since the number of RB subcarriers (12) is larger than thenumber of such symbols (8), the virtual pattern can be viewed as builtfrom an 8×8 Costas array staggered on the top with an array of size 4×8.

FIG. 8B depicts the exemplary virtual pattern, where “−” corresponds toREs where PRS is transmitted on antenna port R6 with “zero” power (whichin practice means any transmit power level significantly lower than thereference power level, since a transmitter may be not able to transmitzero power with its power amplifier being ON).

FIG. 8C illustrates an example of how the virtual pattern can beconstructed from a Costas array, where only symbols with PRS REs areselected from FIG. 8A, the white area includes subcarriers that form aCostas array, and the gray area includes repeated rows from that array.It will be understood that the power levels can be set in various otherways to achieve the same goal.

Thus, it will be understood that a PRS pattern should be considered asmore than just a set of REs of the same power level. This is differentfrom the current standard, for example, which provides that within apositioning occasion (i.e., N consecutive downlink positioningsubframes), the transmission power of PRS is the same in all PRS REsover all positioning subframes and across the entire PRS transmissionbandwidth (which can be smaller than the system bandwidth).

The inventors' solutions described above can either be applied in anetwork node or in a UE as explained below.

PRS Pattern Modification/Shaping by Network Node

PRS having patterns as described above are transmitted by an eNodeB likeother signals, but with appropriate power levels applied to enableshaping or with modified patterns obtained by shifting. PRS patternshaping in the network can be done in a distributed, or localized,method or a centralized, or coordinated, method.

Distributed or Localized Method

The PRS power optimization problem detailed above can be solved locallyin each eNodeB. One approach is an algorithm for PRS pattern shaping(i.e., controlling PRS transmit power according to Eq. 10) isimplemented in an eNodeB. Power levels can be pre-configured orotherwise set, e.g., based on PCI.

Central or Coordinated Method

To maintain good interference coordination on PRS signals, a centralizedpower optimization approach is currently preferred, where a suitableunit, such as a positioning node 160 (e.g., an E-SMLC in LTE or anotherdevice that may have an interface with an E-SMLC) decides the PRS powersin a centralized manner for all or at least a subset of eNodeBs whilecoordinating interference among eNodeBs by adding an additionalconstraint to each per-eNodeB PRS power optimization problem. This meansthat local PRS power optimization problems are combined into a globaloptimization problem where PRS powers are decided simultaneously formultiple cells by the positioning node. Hence the recommended transmitpower of the PRS pattern of a particular eNodeB is signaled to theeNodeB over the interface between the positioning node and the eNodeB,e.g., according to the LTE Positioning Protocol Annex (LPPa) protocol,which is described for example in 3GPP TS 36.455. Thus, sending anindication, or signaling, from a network node, such as an eNodeB or apositioning node in the core network, that the node implements a methoddescribed in this application is thus an embodiment of this invention.

To simplify UE implementation, which searches for the PRS from differentcells, it is preferred to signal the relevant PRS transmit power(s) tothe UE over the radio interface from a suitable network node, e.g., froman E-SMLC to a UE over the LPP protocol or from an eNodeB to a UE usingthe RRC protocol, which is described for example in 3GPP TS 36.331.Signaling the relevant PRS transmit power levels to a UE is thus anembodiment of this invention. In a currently preferred solution, thepower levels can be encoded and a unique mapping between R_(k) andP_(l,k) is assumed, so there is no need to signal power levels for allPRS REs in the pattern, assuming the encoding table and the mapping areknown to the UE.

Zero power levels can be indicated as muted PRS REs. The mutingconfiguration can be signaled from the positioning node (e.g., anE-SMLC) to the UE using LPP messaging. The smallest muted element iscurrently a positioning occasion, which is a set of up to sixconsecutive positioning subframes, and the pattern is signaled as abinary string of one of several pre-defined lengths. For example, thestring 0101 can indicate to the UE that PRS are muted every secondpositioning occasion starting from the first one. It will be understoodthat another indicator could be used, e.g., to indicate whether or notmuting is used within a pattern (assuming that if muted, then always thesame REs are muted or the group of muted REs can be found out, e.g.,from the PCI) or an explicit muting pattern describing all PRS REs canbe sent, e.g., as a binary string of 12×8 bits for symbols having thenormal cyclic prefix.

FIG. 9A is a flow chart of a method of generating reference signals,such as PRS, in accordance with this invention as described above. Instep 902, a predetermined reference signal such as the currentlyspecified PRS is generated by a suitable electronic processor circuit orretrieved from a suitable electronic memory, which can be part of theprocessor circuit. The pre-determined reference signals are typicallystored in a memory and retrieved by the processor as described below. Instep 904, indicated by the dashed lines, the predetermined referencesignal is modified (step 906) or pattern-shaped (step 908), resulting inone or more reference signals as described above. It can be noted that a“modified” PRS can be considered a PRS that has one or more changed REsin comparison to the currently specified PRS, and that a “shaped” PRScan be considered a PRS that has its signal level adjusted beforecorrelation with a replica of the PRS.

FIG. 9B is a flow chart of a method of generating reference signals,such as PRS, by modifying a predetermined reference signal in accordancewith this invention as described above. In step 902, a predeterminedreference signal such as the currently specified PRS is generated by asuitable electronic processor circuit or retrieved from a suitableelectronic memory, which can be part of the processor circuit. In step904, indicated by the dashed lines, the predetermined reference signalis modified by cyclically shifting selected REs (step 906-1) in thepredetermined reference signal. As described above, REs are selected andshifted such that one or more of the following occurs: empty subcarriersin the predetermined PRS are filled with at least one RE; PRS REs arerearranged such that the PRS RE density over coherently accumulatedsegments (e.g., all symbols within a RB) is more uniform and preferablyas uniform as possible; and the same frequency reuse as in thestandardized patterns is maintained over all symbols where PRS istransmitted.

FIG. 9C is a flow chart of a method of generating reference signals,such as PRS, by applying signal shaping to a predetermined referencesignal in accordance with this invention as described above. In step902, a predetermined reference signal such as the currently specifiedPRS is generated by a suitable electronic processor circuit or retrievedfrom a suitable electronic memory, which can be part of the processorcircuit. In step 904, indicated by the dashed lines, the predeterminedreference signal is shaped by adjusting the transmitted and/or receivedpower levels of selected REs (step 908-1) in the predetermined referencesignal. Adjusting the transmitted power level can be carried out by aneNodeB as described above, and adjusting the received power level can becarried out by a UE as described above. When the adjustment is carriedout by the UE, the adjustment is implemented before the correlator inthe UE. As described above, REs in the predetermined reference signalare selected and power levels are adjusted such that the autocorrelationof the resulting reference signal is optimized, for example according toEq. 5 to maximize the ratio of the squared power of the mainautocorrelation peak to the total energy of the autocorrelation or toEq. 6 to maximize the ratio of a metric that corresponds to the energyin the main autocorrelation peak accumulated over a number of samples(slots) to the total energy of the autocorrelation.

FIG. 10 is a block diagram of an example of a portion of an eNodeB 130or similar transmitter for a communication system 100 that uses thereference signals described above. Several parts of such a transmitterare known and described for example in Clauses 6.3 and 6.4 of 3GPP TS36.211 cited above. Reference signals having symbols as described aboveare produced by a suitable generator 131 and provided to a modulationmapper 133 that produces complex-valued modulation symbols. A layermapper 135 maps the modulation symbols onto one or more transmissionlayers, which generally correspond to antenna ports as described above.An RE mapper 137 maps the modulation symbols for each antenna port ontorespective REs, and an OFDM signal generator 139 produces one or morecomplex-valued time-domain OFDM signals for eventual transmission.

It will be appreciated that the functional blocks depicted in FIG. 10can be combined and re-arranged in a variety of equivalent ways, andthat many of the functions can be performed by one or more suitablyprogrammed digital signal processors. Moreover, connections among andinformation provided or exchanged by the functional blocks depicted inFIG. 10 can be altered in various ways to enable a device to implementthe methods described above and other methods involved in the operationof the device in a digital communication system.

PRS Pattern Modification/Shaping by UE

In another embodiment of the invention, generating modified or shapedPRS patterns (i.e., power reduction of the PRS pattern) is done by theUE, preferably before the correlator in the UE receiver. It is currentlypreferred that if power shaping is done in the UE, power shaping shouldnot be done in the network.

One alternative is that by default the UE always performs PRS patternshaping. This can be ensured by specifying requirements and test cases.For example, the OTDOA positioning requirements can be set such that therequired accuracy can be achieved only if the PRS correlation propertiesare improved, thus forcing the UE to apply the shaping. Test cases canbe specified to verify those requirements. The test setup is such that aUE will pass the test only if the UE performs the PRS pattern shaping.

Another alternative is that both eNodeB- and UE-based solutions areused, but only one is to be used at a time. Hence in this case a UEneeds to be informed by the network whether the PRS shaping is done bythe network or not, e.g., by signaling one bit. In case no PRS shapingis done by the network, the UE can perform it. Yet another alternativeis that the network node explicitly commands or requests the UE when toperform PRS pattern shaping.

It will be appreciated that a UE-based solution can be easier to controland require less signaling, which uses system capacity. On the otherhand, if a UE is not certain that PRS shaping is applied at the networkside, the UE can make wrong decisions while performing measurements.From the point of view of system performance, it is currently believedthat the shaping can be implemented at either the eNodeB side or the UEside.

It will also be appreciated that if an eNodeB is transmitting modifiedPRS into its cell that includes several UEs, then it is not necessaryfor all of the UEs to operate in the same way, i.e., without powershaping. PRS patterns modified by shifting may not be optimal, and sofurther modification by power shaping can be applied. The eNodeB canapply the power shaping, or those UEs that are capable of doing so canapply the power shaping. Even if the eNodeB PRS transmit power level issignaled to the UEs, all of the UEs in the eNodeB's cell do not have tooperate in the same way.

FIG. 11 is a block diagram of an arrangement 500 in a UE 110 that canimplement the methods described above. It will be appreciated that thefunctional blocks depicted in FIG. 11 can be combined and re-arranged ina variety of equivalent ways, and that many of the functions can beperformed by one or more suitably programmed digital signal processors.Moreover, connections among and information provided or exchanged by thefunctional blocks depicted in FIG. 11 can be altered in various ways toenable a UE to implement other methods involved in the operation of theUE.

As depicted in FIG. 11, a UE receives a DL radio signal, including forexample synchronization signals, CRS, and PRS, through an antenna 502and typically down-converts the received radio signal to an analogbaseband signal in a front end receiver (FE RX) 504. The baseband signalis typically spectrally shaped by one or more suitable filters andconverted from analog form to digital form by a suitableanalog-to-digital converter (not shown in FIG. 11).

The time-domain received signal generated by the FE RX 504 is providedto a processor 506 that typically includes a programmable processor orcircuit that implements an FFT or other suitable algorithm and generatesa frequency-domain (spectral) representation of the baseband receivedsignal. The spectral representation is provided to other processescarried out by the UE and to a suitable correlator 508 that isconfigured to compare the received signal to a locally generated replicaof the PRS or another signal known by the UE. As shown, the correlatoris configured to operate on the received signal in the frequency domain,but it will be understood that the arrangement 500 can be configuredsuch that the correlator operates on the received signal in the timedomain. A correlation signal generated by the correlator 508 is providedto a suitable timing processor 510 that is configured to determine timeinstants of correlation peaks and other events of interest.

The processor 506, correlator 508, and processor 510 communicate withand are controlled by a control unit 512 that communicates with a memoryunit 513, and are configured to handle PRS signals as described above ineither the time domain or the frequency domain. As described above, thememory unit 513 can store predetermined PRS and provide the stored PRSfor use by the control unit 512, and the memory unit 513 can simply bepart of the control unit 512 (or for that matter in the processor 506,correlator 508, and/or the processor 510). For example, the control unit512 can be configured such that the received signal power is modified inthe frequency domain (power shaping) by suitably adjusting the signalprovided by the processor 506 to the correlator 508. In this way,UE-side PRS power shaping can be implemented. The control unit 512 candetermine the PRS replica used by the correlator 508 (and the PRSincluded in the received signal) in any suitable way, e.g., from OTDOAassistance data or other data received by the UE from an eNodeB.

The control unit 512 is further configured such that timing measurementsgenerated by the processor 510 are provided to a transmit signalgenerator 514, which formats the information in a suitable way intomessages and an UL signal that is provided to a transmitter front-end(FE TX) 516, which up-converts the signal and transmits the up-convertedsignal through the antenna 502 to an eNodeB 130. In particular, thetiming measurements are signaled to the positioning node (forUE-assisted positioning) or used locally by the UE (e.g., for UE-basedpositioning).

In the arrangement depicted in FIG. 11, the control unit 512 keeps trackof substantially everything needed to configure the processors 506, 510,correlator 508, and generator 514. This can include cell identity (forreference signal extraction and cell-specific scrambling of referencesignals). Communication between the processor 506 and control unit 512also includes, for example, cyclic prefix configuration. Although thecorrelator 508 and processor 510 are depicted in FIG. 11 as separateblocks, it will be appreciated that the control unit 512 can include acorrelator or implement a correlator function, and can determine thetiming information needed for UE positioning as described above.

The control unit and other blocks of the UE can be implemented by one ormore suitably programmed electronic processors, collections of logicgates, etc. that processes information stored in one or more memories.The stored information can include program instructions and data thatenable the control unit to implement the methods described above. Itwill be appreciated that the control unit typically includes timers,etc. that facilitate its operations.

It will be appreciated that the methods and devices described above canbe combined and re-arranged in a variety of equivalent ways, and thatthe methods can be performed by one or more suitably programmed orconfigured digital signal processors and other known electronic circuits(e.g., discrete logic gates interconnected to perform a specializedfunction, or application-specific integrated circuits). Many aspects ofthis invention are described in terms of sequences of actions that canbe performed by, for example, elements of a programmable computersystem. UEs embodying this invention include, for example, mobiletelephones, pagers, headsets, laptop computers and other mobileterminals, and the like. Moreover, this invention can additionally beconsidered to be embodied entirely within any form of computer-readablestorage medium having stored therein an appropriate set of instructionsfor use by or in connection with an instruction-execution system,apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch instructionsfrom a medium and execute the instructions.

Among the many advantages provided by this invention, there are theimproved correlation properties of PRS patterns, and pattern shaping,which is a flexible way of creating virtual PRS patterns, for example,over those currently defined by the 3GPP standard. Pattern shaping canactually be applied to any other reference signal (which needs to betransmitted with a relatively sparse pattern). Pattern shaping by poweradjustment also enables frequency control on the measured referencesignals—a convenient network-adaptive approach, especially because othertransmissions are in general not affected if the pattern shaping is donewithin positioning subframes (or equivalent assumed to below-interference subframes without data transmissions). Pattern shapingby power adjustment (at the network side or the UE side) can also beapplied to reference signals other than PRS. Muting on the subframe,symbol, or RE level are all special cases of the pattern shapingapproach, if done by the network, just with extra constraints to ensurethe desired interference management strategy. Different power levels ofreference signals used for positioning can also be allowed within apositioning subframe in accordance with this invention, although it willbe understood that such different power levels are not currentlyspecified by 3GPP TS 36.213.

It will be appreciated that procedures described above are carried outrepetitively as necessary, for example, to respond to the time-varyingnature of communication channels between transmitters and receivers. Inaddition, it will be understood that the methods and apparatus describedhere can be implemented in various system nodes.

To facilitate understanding, many aspects of this invention aredescribed in terms of sequences of actions that can be performed by, forexample, elements of a programmable computer system. It will berecognized that various actions could be performed by specializedcircuits (e.g., discrete logic gates interconnected to perform aspecialized function or application-specific integrated circuits), byprogram instructions executed by one or more processors, or by acombination of both. Wireless devices implementing embodiments of thisinvention can be included in, for example, mobile telephones, pagers,headsets, laptop computers and other mobile terminals, base stations,and the like.

Moreover, this invention can additionally be considered to be embodiedentirely within any form of computer-readable storage medium havingstored therein an appropriate set of instructions for use by or inconnection with an instruction-execution system, apparatus, or device,such as a computer-based system, processor-containing system, or othersystem that can fetch instructions from a storage medium and execute theinstructions. As used here, a “computer-readable medium” can be anymeans that can contain, store, or transport the program for use by or inconnection with the instruction-execution system, apparatus, or device.The computer-readable medium can be, for example but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device. More specific examples (anon-exhaustive list) of the computer-readable medium include anelectrical connection having one or more wires, a portable computerdiskette, a random-access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), and anoptical fiber.

Thus, the invention may be embodied in many different forms, not all ofwhich are described above, and all such forms are contemplated to bewithin the scope of the invention. For each of the various aspects ofthe invention, any such form may be referred to as “logic configured to”perform a described action, or alternatively as “logic that” performs adescribed action.

What is claimed is:
 1. A method of using reference signals (RS) in anorthogonal frequency division multiplex communication system in whichthe RS are organized in a RS pattern of resource elements (REs) thatincludes at least one of: a first plurality of columns corresponding tosymbols and a second plurality of rows corresponding to subcarriers, anda set of power levels of RS REs, the method comprising: forming amodified RS pattern based on a predetermined RS pattern by at least oneof: assigning respective different transmission power levels to selectedREs of the predetermined RS pattern; and adjusting received signal powerlevels of selected REs of the predetermined RS pattern; wherein REs areselected and power levels are assigned or adjusted for maximizing a mainpeak of an autocorrelation of a received signal including the modifiedRS pattern.
 2. The method of claim 1, wherein the main peak is maximizedby maximizing either a ratio of a squared power of a mainautocorrelation peak to a total energy of the autocorrelation or a ratioof a metric that corresponds to an accumulated energy in the mainautocorrelation peak to the total energy of the autocorrelation.
 3. Themethod of claim 1, wherein forming the modified RS pattern includescyclically shifting REs in at least one column of the predetermined RSpattern; and REs in the at least one column of the predetermined RSpattern are cyclically shifted such that a number of REs per column anda space between REs in a column of the predetermined RS pattern is aboutthe same as a number of REs per column and a space between REs in acolumn of the modified RS pattern.
 4. The method of claim 1, wherein atleast one empty subcarrier of the predetermined RS pattern is filledwith an RE of the modified RS pattern.
 5. The method of claim 1, whereina density of RS REs over coherently accumulated segments of the modifiedRS pattern either is more uniform among subcarriers or varies moreregularly among subcarriers relative to a density of RS REs overcoherently accumulated segments of the predetermined RS pattern.
 6. Themethod of claim 1, further comprising sending, to another network node,an indication whether the method is implemented in a network node in thecommunication system.
 7. The method of claim 1, wherein an indication ofthe power levels is sent either to at least one of a base station and apositioning node, or to a user equipment according to either a Long TermEvolution Positioning Protocol or a Radio Resource Control protocol. 8.The method of claim 7, wherein the indication is included in ObservedTime Difference of Arrival assistance data.
 9. The method of claim 1,wherein the selected REs are selected by either an eNodeB or apositioning node in the communication system.
 10. The method of claim 1,wherein REs are selected by a receiving node configured to reshapereceived signals before correlating the received signals with a localreplica of the modified RS pattern.
 11. A reference signal generator inan orthogonal frequency division multiplex communication system in whichreference signals (RS) are organized in a RS pattern of resourceelements (REs) that includes at least one of: a first plurality ofcolumns corresponding to symbols and a second plurality of rowscorresponding to subcarriers, and a set of power levels of RS REs, thegenerator comprising: an electronic processor configured to form amodified RS pattern based on a predetermined RS pattern by at least oneof: assigning respective different transmission power levels to selectedresource elements of the predetermined RS pattern; and adjustingreceived signal power levels of selected REs of the predetermined RSpattern; wherein REs are selected and power levels are assigned oradjusted for maximizing a main peak of an autocorrelation of a receivedsignal including the modified RS pattern.
 12. The generator of claim 11,wherein the main peak is maximized by maximizing either a ratio of asquared power of a main autocorrelation peak to a total energy of theautocorrelation or a ratio of a metric that corresponds to anaccumulated energy in the main autocorrelation peak to the total energyof the autocorrelation.
 13. The generator of claim 11, wherein themodified RS pattern is formed by cyclically shifting REs in at least onecolumn of the predetermined RS pattern; and REs in the at least onecolumn of the predetermined RS pattern are cyclically shifted such thata number of REs per column and a space between REs in a column of thepredetermined RS pattern is about the same as a number of REs per columnand a space between REs in a column of the modified RS pattern.
 14. Thegenerator of claim 11, wherein at least one empty subcarrier of thepredetermined RS pattern is filled with an RE of the modified RSpattern.
 15. The generator of claim 11, wherein a density of RS REs overcoherently accumulated segments of the modified RS pattern either ismore uniform among subcarriers or varies more regularly amongsubcarriers relative to a density of RS REs over coherently accumulatedsegments of the predetermined RS pattern.
 16. The generator of claim 11,wherein the electronic processor is further configured to indicate thepower levels to another node in the communication system.
 17. Thegenerator of claim 16, wherein the indication is sent either to at leastone of a base station and a positioning node, or to a user equipmentaccording to either a Long Term Evolution Positioning Protocol or aRadio Resource Control protocol.
 18. The generator of claim 17, whereinthe indication is included in Observed Time Difference of Arrivalassistance data.
 19. The generator of claim 11, wherein the generator isincluded in either an eNodeB or a positioning node in the communicationsystem.
 20. An apparatus for a user equipment in an orthogonal frequencydivision multiplex communication system for using reference signals (RS)organized in a RS pattern of resource elements (REs), comprising: acorrelator; a modified RS pattern generator configured to generate amodified RS pattern based on a predetermined RS pattern that is includedin a received signal, wherein the modified RS pattern includes selectedREs of the predetermined RS pattern that have adjusted signal powerlevels; and an electronic processor configured to modify the receivedsignal according to the modified RS pattern and form a modified receivedsignal; wherein the correlator is configured to correlate the modifiedreceived signal and the modified RS pattern and form a correlationresult; and REs are selected and power levels are adjusted formaximizing a main peak of an autocorrelation of a received signalincluding the modified RS pattern.
 21. The apparatus of claim 20,wherein REs are selected and power levels are adjusted such that anautocorrelation of the modified RS pattern is optimized by maximizingeither a ratio of a squared power of a main autocorrelation peak to atotal energy of the autocorrelation or a ratio of a metric thatcorresponds to an accumulated energy in the main autocorrelation peak tothe total energy of the autocorrelation.
 22. The apparatus of claim 20,wherein the RS are positioning reference signals.
 23. The apparatus ofclaim 20, wherein the apparatus is included in a user equipment.